Portable computing devices, for example Portable Navigation Devices (PNDs), which include GPS (Global Positioning System) signal reception and processing functionality are well known and are widely employed as in-car or other vehicle navigation systems.
In general terms, a modern PND comprises a processor, memory, and map data stored within said memory. The processor and memory cooperate to provide an execution environment in which a software operating system can be established, and additionally it is commonplace for one or more additional software programs to be provided to enable the functionality of the PND to be controlled, and to provide various other functions.
Typically these devices further comprise one or more input interfaces that allow a user to interact with and control the device, and one or more output interfaces by means of which information may be relayed to the user. Illustrative examples of output interfaces include: a visual display and a speaker for audible output. Illustrative examples of input interfaces include: one or more physical buttons to control on/off operation or other features of the device (which buttons need not necessarily be on the device itself but could be on a steering wheel if the device is built into a vehicle), and a microphone for detecting user speech. In one particular arrangement, the output interface display may be configured as a touch sensitive display (by means of a touch sensitive overlay or otherwise) additionally to provide an input interface by means of which a user can operate the device through the display.
Devices of this type will also often include one or more physical connector interfaces by means of which power and optionally data signals can be transmitted to and received from the device, and optionally one or more wireless transmitters/receivers to allow communication over cellular telecommunications and other signal and data networks, for example Bluetooth, Wi-Fi, Wi-Max, GSM, UMTS and the like.
PNDs of this type also include a GPS antenna by means of which satellite-broadcast signals, including location data, can be received and subsequently processed to determine a current location of the device.
The PND may also include electronic gyroscopes and accelerometers which produce signals that can be processed to determine the current angular and linear acceleration, and in turn, and in conjunction with location information derived from the GPS signal, velocity and relative displacement of the device and thus the vehicle in which it is mounted. Typically, such features are most commonly provided in in-vehicle navigation systems, but may also be provided in PNDs if it is expedient to do so.
The utility of such PNDs is manifested primarily in their ability to determine a route between a first location (typically a start or current location) and a second location (typically a destination). These locations can be input by a user of the device, by any of a wide variety of different methods, for example by postcode, street name and house number, previously stored “well known” destinations (such as famous locations, municipal locations (such as sports grounds or swimming baths) or other points of interest), and favourite or recently visited destinations.
Typically, the PND is enabled by software for computing a “best” or “optimum” route between the start and destination address locations from the map data. A “best” or “optimum” route is determined on the basis of predetermined criteria and need not necessarily be the fastest or shortest route. The selection of the route along which to guide the driver can be very sophisticated, and the selected route may take into account existing, predicted and dynamically and/or wirelessly received traffic and road information, historical information about road speeds, and the driver's own preferences for the factors determining road choice (for example the driver may specify that the route should not include motorways or toll roads).
PNDs of this type may typically be mounted on the dashboard or windscreen of a vehicle, but may also be formed as part of an on-board computer of the vehicle radio or indeed as part of the control system of the vehicle itself. The navigation device may also be part of a hand-held system, such as a PDA (Portable Digital Assistant), a media player, a mobile phone or the like, and in these cases, the normal functionality of the hand-held system is extended by means of the installation of software on the device to perform both route calculation and navigation along a calculated route.
In the context of a PND, once a route has been calculated, the user interacts with the navigation device to select the desired calculated route, optionally from a list of proposed routes. Optionally, the user may intervene in, or guide the route selection process, for example by specifying that certain routes, roads, locations or criteria are to be avoided or are mandatory for a particular journey. The route calculation aspect of the PND forms one primary function, and navigation along such a route is another primary function.
During navigation along a calculated route, it is usual for such PNDs to provide visual and/or audible instructions to guide the user along a chosen route to the end of that route, i.e. the desired destination. It is also usual for PNDs to display map information on-screen during the navigation, such information regularly being updated on-screen so that the map information displayed is representative of the current location of the device, and thus of the user or user's vehicle if the device is being used for in-vehicle navigation.
An icon displayed on-screen typically denotes the current device location, and is centred with the map information of current and surrounding roads in the vicinity of the current device location and other map features also being displayed. Additionally, navigation information can be displayed, optionally in a status bar above, below or to one side of the displayed map information, an example of the navigation information includes a distance to the next deviation from the current road required to be taken by the user, the nature of that deviation possibly being represented by a further icon suggestive of the particular type of deviation, for example a left or right turn. The navigation function also determines the content, duration and timing of audible instructions by means of which the user can be guided along the route. As can be appreciated, a simple instruction such as “turn left in 100 m” requires significant processing and analysis. As previously mentioned, user interaction with the device may be by a touch screen, or additionally or alternately by steering column mounted remote control, by voice activation or by any other suitable method.
In addition, the device may continually monitor road and traffic conditions, and offer to or choose to change the route over which the remainder of the journey is to be made due to changed conditions. Real time traffic monitoring systems, based on various technologies (e.g. mobile phone data exchanges, fixed cameras, GPS fleet tracking) are being used to identify traffic delays and to feed the information into notification systems, for example a Radio Data System (RDS)-Traffic Message Channel (TMC) service.
Whilst it is known for the device to perform route re-calculation in the event that a user deviates from the previously calculated route during navigation (either by accident or intentionally), a further important function provided by the device is automatic route re-calculation in the event that real-time traffic conditions dictate that an alternative route would be more expedient. The device is suitably enabled to recognize such conditions automatically, or if a user actively causes the device to perform route re-calculation for any reason.
It is also known to allow a route to be calculated with user defined criteria for example, the user may wish to avoid any roads on which traffic congestion is likely, expected or currently prevailing. The device software would then calculate various routes using stored information indicative of prevailing traffic conditions on particular roads, and order the calculated routes in terms of level of likely congestion or delay on account thereof. Other traffic information-based route calculation and navigation criteria are also possible.
Hence, it can be seen that traffic related information is of particular use when calculating routes and directing a user to a location. In this respect, and as mentioned above, it is known to broadcast traffic-related information using the RDS-TMC facility supported by some broadcasters. In the UK, for example, one known traffic-related information service is broadcast using the frequencies allocated to the station known as “Classic fm”. The skilled person should, of course, appreciate that different frequencies are used by different traffic-related information service providers.
A PND, provided with an RDS-TMC receiver for receiving RDS data broadcast, can decode the RDS data broadcast and extract TMC data included in the RDS data broadcast. Such Frequency Modulation (FM) receivers need to be sensitive. For many PNDs currently sold, an accessory is provided comprising an RDS-TMC tuner coupled to an antenna at one end and a connector at another end thereof for coupling the RDS-TMC receiver to an input of the PND.
Devices of the type described above, for example the 920 GO model manufactured and supplied by TomTom International which employ the above-described antenna, support a process of enabling users to navigate from one position to another, in particular using traffic-related information. Such devices are of great utility when the user is not familiar with the route to the destination to which they are navigating.
However, the effectiveness of such devices can sometimes depend upon the antenna structure employed. In this respect, in the field of antenna design, a number of antenna structures are known to have varying degrees of suitability in relation to receipt of RDS-TMC data. One antenna structure is a so-called dipole antenna structure, having numerous variants thereof, for example a symmetric dipole antenna structure and an asymmetric dipole antenna structure. Wired variants of the symmetric and asymmetric dipole antenna structures comprise a pair of wires, for example flexible wires, constituting a first pole and a second pole. The symmetric antenna structure was originally designed for symmetric Radio-Frequency (RF) input circuits, the symmetric antenna structure simply comprising symmetric twin cables that were connected to an RF receiver. An RF transformer was provided in the RF receiver in order to convert a symmetric antenna signal to an asymmetric antenna signal that could be amplified by a suitable RF amplifier circuit in the RF receiver. Over time, as this technology was developed, a so-called “feedline” was introduced into the design of the antenna for high frequency and/or weak signal applications in order to distance the antenna poles from “noisy” electrical circuitry to which the antenna structure was to be coupled. One type of feedline employed was in the form of a length of coaxial cable. However, the coaxial cable is a transmission line having conductors of unequal impedances with respect to ground potential and so is considered “unbalanced”. In order to match the symmetric impedances (balanced) of the pole wires with the asymmetric impedances of the feedline, it is known to place a so-called “balun” in-line between the pole wires and the feedline, thereby matching the impedances of the pole wires and the feedline and so mitigating unwanted common-mode currents from flowing in the feedline that can cause the pole wires to radiate RF energy.
Unfortunately, despite the distancing of the poles provided by the coaxial feedline, the antenna structure comprising the pole wires and the coaxial feedline of the type described above is still susceptible to Electromagnetic Interference (EMI) from neighbouring electrical and/or electronic devices, for example the PND and/or a power supply, for example a Cigarette Lighter Adaptor (CLA). In this respect, unlike electronic systems integrated into a vehicle, for example an automobile, the PND is “floating” with respect to ground at radio frequencies and so received signals are not referenced to an “EMI clean” body of the vehicle, but to a “noisy” ground reference of the PND instead. Furthermore, it is undesirable, from the perspective of a manufacturer of a PND, to require a user of the PND to connect an antenna to the body of the vehicle in order to obtain the desired “clean” ground reference. Even if the distance provided by the coaxial feedline is taken into account, the antenna is nevertheless still positioned very close to the EMI “noisy” PND. Consequently, antenna performance can, in some circumstances, be inadequate resulting in the PND not receiving any data or only partial data. From the perspective of a user of the PND, the user simply perceives that no or incomplete traffic information is available and can wrongly conclude that the PND and/or the TMC accessory are/is malfunctioning.
European patent publication no. EP 1 672 787 relates to a broadcast receiver having an antenna socket coupled to a common mode input filter of a radio tuner via a feeder line. However, the input filter requires a ground, which is provided by the radio tuner. An interference-free analogue to the ground is not, unfortunately, available in the context of the RDS-TMC tuner and antenna.
Other solutions to reduce influence of externally interfering sources of RF signals are known. For example, external sources capable of emitting electromagnetic radiation can be shielded in respect of certain frequency ranges. However, such solutions are expensive and can result in other problems relating to, for example, heat dissipation. Additionally, when circuit designs change, provisions made for electromagnetic shielding can require modification too. Hence, design and implementation costs and lack of re-usability of an electromagnetic radiation shielding solution makes electromagnetic shielding of the external sources of electromagnetic radiations undesirable.
Due to the presence of the above-described unwanted EMI, a combination of a desired RF signal and an unwanted EMI signal is received at an input of an RF receiver. Whilst it is possible to increase sensitivity of the RF receiver, increased sensitivity does not serve to increase a Signal-to-Noise Ratio (SNR) of the RF receiver and hence the process of discriminating the wanted signal from the unwanted signal.